The present invention relates generally to novel construction for a dosimeter badge, and to methods of producing said dosimeter badge construction, and relates more specifically to a novel dosimeter badge formed on flat stock and to a novel method of producing dosimeter badges using flat stock.
In view of the fact that exposure to an excessive level of radiation can be extremely harmful, many employers whose employees must work in a radioactive environment, such as nuclear power plant operators, utilize a program whereby the employees are required to wear one or more dosimeter badges while at work. After a period of time, the dosimeter badges are collected and analyzed to determine the extent of radiation exposure to which each employee has been subject. Thereafter, corrective measures can be taken to diminish the risk of any particular employee of overexposure to radiation.
Presently, there are four methods of dosimetry which are generally used to monitor the extent of exposure to radiation. The first method is the use of radiological monitoring film. Radiological film has been used to monitor radioactive exposure in the workplace for over seventy years. In fact, this method remains the most widely used in the world. Essentially, when radiological film is used, each worker is required to wear one or more dosimeter badges in each of which sits radiological film. After a period of time, the badges are collected and analyzed to determine the amount of radioactive exposure.
As radiation passes through a dosimeter badge, filters in the badge filter the radiation in order to produce a multiple-density image on the radiological film. This multiple-density image is analyzed and provides, essentially, a quantitative and visual record of both the amount of exposure, and the conditions that existed during the exposure. For example, the greater the density or film darkening on the radiological film, the greater the dose of radiation to which the radiological film was exposed. Additionally, the angularization of the image formed indicates direction of exposure or movement or lack thereof, during exposure. For example, a sharp image formed on the radiological film indicates that the exposure to radiation was static, such as would be the case if the badge were left in an x-ray examination room. In contrast, a blurred image formed on the radiological film indicates that the badge was moving during exposure. Other characteristics of the image formed on the radiological film may indicate that the badge was incorrectly worn, or that the film had been contaminated.
The image formed on the radiological film not only provides a visual record of the exposure, but because of the nature of radiological film, provides a permanent record of the exposure, that can be re-evaluated should the need arise. Unfortunately, radiological film cannot be reused, however, it is usually more desirable to store the film to maintain a permanent record of the exposure. Typically, each radiological film includes embossed characters or coded perforations thereon to allow each film to be identified in terms of who wore the badge in which that particular film was contained, and during what specific period of time.
While radiological film is relatively inexpensive due to economies of scale, radiological film presents some disadvantages. For example, elaborate packaging must be used to protect the emulsion on the film from light, humidity and handling damage. This is because light, heat and pressure may induce the film to darken, and this film darkening can be mistaken for exposure to radiation. Furthermore, radiological film can be used only to monitor radiation exposure within a specific, limited range. Additionally, developing the film presents a laboratory inconvenience. Unfortunately, automated processors typically found in hospitals cannot be used to develop the film because these processors are designed for much larger film and rapid processing. Developing the film requires close monitoring of chemical strength and temperature, as well as developing time. For these reasons, developing the film and analyzing the image thereon is generally left to large commercial monitoring services which can employ advanced quality control methods, and which can benefit from economies of scale.
A common badge in which radiological film is inserted is a badge which includes a plastic film-holding member having a slot thereon for receiving the film. Adjacent to and surrounding the slot are two U-shaped, usually metal, filters, wherein each U-shaped filter is formed of a different metal having a different atomic number. Additionally, the plastic film-holding member typically has an aperture therein that leads to the film-receiving slot, and therefore to the film. Each of the U-shaped filters and the apertures are located adjacent to a different portion of the film when the film is inserted in the slot. This overall construction of the dosimeter badge provides essentially four different filters positioned adjacent the film for filtering radiation that passes through the badge. Specifically, a first filter of metal, a second filter of another type of metal, a third filter of plastic (provided by the plastic film-holding member itself), and a fourth "filter", a non-filter, formed by the aperture adjacent the slot. As a result of the four filters, a multiple-density, or "shaped", image is formed on the film when radiation passes through the badge. Subsequently, this multiple-density image can be analyzed to determine the exposure to radiation.
Unfortunately, the above-described badge used in connection with radiological film is bulky and can only be used in connection with film. The badge cannot be used in connection with the other methods of dosimetry which will be described. Additionally, the construction of the badge is such that the film is inserted into the plastic film-holding member as a secondary operation, normally by hand. Furthermore, the film must be removed from the badge in order to analyze the image formed thereon, and this requires yet another operation.
The remaining methods of dosimetry utilize special crystals doped with impurities which trap energy deposited by radiation. When these special crystals are used to monitor exposure to radiation in the workplace, each worker is required to wear one or more dosimeter badges in each of which sits a plurality, such as four, of the crystals. As radiation passes through a badge, filters associated with certain of the crystals, filter the radiation as the radiation deposits energy in each of the four crystals, one crystal remaining unfiltered. After some period of time, the dosimeter badges are collected, and the crystals are analyzed to determine the extent of exposure to radiation.
Within one method of dosimetry, the crystals are analyzed by heating them to high temperatures, such as from 250.degree. to 300.degree. degrees Celsius, causing the energy trapped in the crystals to be released as luminescence. The amount of luminescence is proportional to the extent of radiation exposure. Therefore, analyzing the amount of luminescence provides that the amount of exposure to radiation can be determined. This method of dosimetry has come to be called thermoluminescence dosimetry (TLD).
Within another method of dosimetry, optical energy is used instead of thermal energy, and specifically laser energy is used to produce the luminescence in the crystals after exposure to radiation. This method of dosimetry has come to be called optically stimulated luminescence (OSL).
Within yet another method of dosimetry, the crystals are cooled with liquid nitrogen, and then stimulated with light. Then, the crystals are allowed to warm to room temperature. During warming, the crystals luminescence in proportion to the amount of energy deposited during exposure to radiation. Therefore, analyzing the luminescence can allow one to determine the extent of exposure to radiation. This method of dosimetry has come to be called cooled optically stimulated luminescence (COSL).
The nature of the special crystals used within the second, third and fourth above-described methods of dosimetry provide certain advantages over radiological film. For example, the measurement range of the crystals greatly exceeds that of film, and the crystals better simulate human tissue than does film. Additionally, the crystals are less susceptible to physical damage. Furthermore, the crystals avoid the chemical developing process required by radiological film, and can be analyzed using a small, highly automated reader.
Unfortunately, the nature of the crystals also offer some disadvantages in relation to film. For example, unlike film, the crystals cannot provide any indication of the exposure conditions. Also, indicia generally cannot be provided on the crystal itself to provide an indication of who wore the badge containing the crystal and during what period of time. Instead, each crystal must be identified by its position in a card or plate that has a unique identification number thereon.
Furthermore, TLD specifically offers additional disadvantages. While the heating of the crystals provide that they can be reused because the dosimetry traps therein are cleared, the clearing of the dosimetry traps provides that the crystals cannot be re-evaluated. Therefore, TLD does not offer the same permanent record of the radiation exposure as does radiological film or the crystals when analyzed using either the OSL or COSL dosimetry methods.
A common dosimeter badge in which the special crystals are inserted is a badge which includes a plastic member that has a slot for receiving a plastic card carrying the four crystals. Once the card is inserted in the slot, a different filter is aligned with the crystals, one crystal remaining unfiltered. A first filter is formed by two metal discs, each comprised of a specific type of metal, and each located on opposing sides of one crystal. A second filter resembles the first, but the discs are formed of a different type of metal, and are aligned with another crystal. A third filter is formed by the plastic member itself, and a fourth "filter", essentially a non-filter, is formed by opposing apertures in the plastic member. After exposure to radiation, the plastic card can be removed from the plastic member, and the crystals can be analyzed using one of the above-described three methods, namely TLD, OSL or COSL. Should TLD be utilized, the crystals will need to be removed from the plastic card before being subjected to the extreme heat required to perform the analysis.
Unfortunately, the above-described dosimeter badge which is used in connection with the crystals and the TLD, OSL and COSL dosimeter methods cannot also be used with radiological film. Additionally, the badge is bulky, and requires the secondary operation of inserting the plastic card in the plastic member, and the subsequent operation of removing the plastic card to analyze the crystals. Additionally, should the TLD dosimetry method be utilized, the crystals must be removed from the plastic card before heating, and the crystals must subsequently be re-associated with some external indicia to identify who wore the corresponding badge and during what time period. Obviously, this presents a chance for error.
While the present invention is not specifically directed to solve all the problems associated with each of the four existing dosimetry methods, the present invention is directed to solve most of the problems encountered heretofore with respect to the badges which have been used in connection therewith. The present invention is also specifically directed to a novel method of producing dosimeter badges.